It is normal practice to produce kaolin pigments by calcination of purified fine particle size hydrous kaolin clay. Calcination of kaolin at temperatures up to about 1100.degree. C. cements particles together and produces products of improved whiteness and opacity. Such pigments are widely used by the paper, plastics, rubber and paint industries. A seminal patent is commonly assigned U.S. Pat. No. 3,586,523, Fanselow et al, the disclosure of which is incorporated herein by cross-reference. This patent is directed to producing low abrasion fine particle size opacifying calcined kaolin clay, such as the pigments supplied under the registered trademarks ANSILEX and ANSILEX 93. Such pigments have an average particle size of about 0.8 microns. When used in paints, the fine calcined pigments obtained by practice of the Fanselow et al patent also provide tint strength and high sheen.
For flat paints, the low angle sheen as measured by 85 degree gloss is generally less than 10. For better flat paints, the 85 degree sheen is 4 or below. To achieve low sheen the particle size of the calcined clays is increased by changing the particle size distribution of the feed such that the feed is coarser than used by Fanselow et al. While a sheen of 4 is achievable by calcining a coarser feed, the hiding and tint strength properties are compromised (reduced). It is extremely difficult to balance the properties of a paint formulation since 85 degree sheen and opacity/tint strength move in opposite direction to their particle size. Thus, as particle size in the optically efficient range increases, opacification decreases. The following commonly assigned patent, U.S. Pat. No. 4,525,818, Kostanzek, discloses means to produce certain coarse calcined kaolin clay pigments especially useful as primary extenders in paint. The calcined clay pigments have an average particle size of about 3 to 4 microns.
Practice of our invention utilizes fluxing compounds during calcination of the kaolin, the compounds being added to hydrous kaolin before calcination. Reference is made to U.S. Pat. No. 2,307,239, Rowland, which is a pioneer patent in the field of calcined kaolin pigments. This patent broadly discloses addition of various alkali and alkaline earth compounds to clay before calcination. A preferred compound is sodium chloride. U.S. Pat. No. 3,853,573, Ferrigno, discloses pigmentary compositions produced by combining a fluxing agent composed of one or a combination of metal oxides with calcined kaolin, with optional hydrous kaolin, and an inorganic binder. Dry addition of flux is practiced by Rowland and Ferrigno; no attempt is made to coarsen the clay in the optically efficient particle size range. In a thesis, "The Kaolin to Mullite Reaction Series," Wilfred Anthony Martinez, Rutgers University, 1979, various "mineralizers" were added to both a fine particle size kaolin and a relatively coarse particle size hydrous kaolin, primarily to explore the effect on the crystallinity. Dry addition of mineralizer was practiced in all experimental work. There is no attempt to control the coarsening of kaolin clay during calcination throughout the optically active size. The phrase "controlled coarsening" as used herein refers to increasing the percentage of coarsened particles throughout the particle size range of 0.5 to 2 microns.
So-called "flash" or "shock" calcination is used to produce relatively coarse kaolin derived pigments. Reference is made to U.S. Pat. No. 3,021,195, Podschus et al. Special calciners must be used and multi-stage thermal treatment is used. The rotary calciners or Nichols furnaces generally used by the kaolin industry cannot be readily reconstructed to function as shock calciners. While the calcined pigments are relatively coarse, average size about 1.4 microns, and sheen is low, the pigments tend to be more yellow than other calcined clay pigments.